:constructor option is given as
(:constructor name arglist ),
then instead of making a keyword-driven constructor function,
defstruct defines a "positional" constructor function,
taking arguments whose meaning is determined by the argument's position
rather than by a keyword. [...]
Because a constructor of this type operates By Order of Arguments, it is sometimes known as a BOA Constructor.
Common Lisp: The Language, 1st edition, pp. 315-316.
Guy L. Steele Jr.